Verifying/calibrating tachometer

What is the best way to check and calibrate a tachometer? I’m highly suspicious of the tach in my ‘89 ski, it reads 4200 RPM at 47 MPH (GPS) with an ACME 470. That seems a bit low based on what I’ve read on here. I know there’s a potentiometer on the back of the tach to adjust the gain, but what’s the best way of obtaining a known good point of comparison? A non-contact photo tach? Digital multimeter with a frequency counter connected to the tach wire (plus math to convert the spark signal to RPM)? I need to buy a tool either way, my multimeter is an analog with no frequency counter, and my timing light does not have a tach either.

Most Tachs have a 4, 6 or 8 cylinder setting on the back of the tach.  Sometimes just moving the switch back and forth fixes the tach.    My 78 Nautique and my 87 Malibu had tach issues at the start of every season.  First couple times out the tach was way off then it started working perfectly the rest of the year.  Eventually the Malibu tach had to be replaced.